1. Field of the Invention
This invention relates to the measurement of inductance generally and, more particularly, to an electronically calibratable proximity, or inductive, switch having a high degree of sensitivity while at the same time being economically manufactured and suitable for use in harsh environments.
2. Background Art
While the present invention has numerous applications, it is especially well suited for use on mobile equipment such as fork lift trucks, manufactured, for example, by Yale Materials Handling Corporation, that may be used in industrial and other environments where the switches may be subjected to mechanical abuse, low and high temperatures, and dirty and abrasive atmospheres.
Typically, such trucks employ a number of mechanical "microswitches" which are activated by the movement of a mechanical part on the truck against a movable arm on the microswitch. The movement of the arm, in turn, effects the closing (or opening) of electrical contacts within the switch, thus providing an indicator signal that the switch has been closed or opened or a control signal as an input to some other device.
Some points of use for microswitches in fork lift trucks are those in connection with the hydraulic control system, platform switches, and brake switches. These microswitches have been found to be troublesome, due to the tolerances in the linkage which activates the switch, the mounting configuration of the switch, the fastening of the wires to the switch, the fact that the switch has contacting and moving parts, and the mechanical tolerances of the inner workings of the switch itself. Further, because a mechanical switch is an easily understood and relatively easily replaced device, it is susceptible to being blamed for most failures, which delays the eventual repair or replacement of the truly defective part.
Proximity switches are known which comprise LRC resonant circuits. When there is no metal near the inductor of the circuit, the circuit gives a high output. When metal comes within a selected distance of the inductor, the metal and the inductor become inductively coupled, the circuit is detuned, and the output of the circuit drops. Another type of known proximity switch is the magnetic switch, such as the reed switch, which is open or closed depending upon its proximity to a magnet. While these known devices are satisfactory in many applications they have a number of disadvantages. The reed switches are generally unsatisfactory in harsh environments, as sharp movements or vibrations can open or close the switches. Resonant circuits require tuning of a capacitance and an inductor and, when a given such switch is installed in a particular piece of equipment, metals other than the actuator may affect performance. Resonant circuits are also prone to electrical noise problems and are sensitive to variations in frequency and voltage. Both types require fairly careful mechanical adjustment and neither can provide multilevel position sensing. Both are relatively expensive and neither can be electronically recalibrated once installed.
Accordingly, it is a principal object of the present invention to provide an inductive switch which is electronically calibratable after installation.
Another object of the invention is to provide an inductive switch which is rugged and may be used in harsh environments with wide temperature variations.
An additional object of the invention is to provide an inductive switch which is highly accurate, yet is economical to manufacture, is easily installed, and requires little maintenance.
It is another object of the present invention to provide an inductive switch such that when a plurality of switches are used in an application, the condition of all such switches may be determined sequentially and rapidly by a single electronic circuit.
It is a further object of the present invention to provide an inductive switch that may be readily mounted in place of a microswitch.
Other objects of the invention, as well as particular features and advantages thereof, will, in part, be apparent and will, in part, be obvious from the following description and the accompanying drawing figures.